Field of the Invention
The invention relates to a circuit arrangement for monitoring temperature of at least one electronic component, which can be impinged with electric current and can be connected to at least one voltage source. Furthermore, the invention relates to a calorimetric mass flowmeter having at least one electronic component, which, in particular, can be impinged with electric current and can be connected to at least one voltage source.
Description of Related Art
In modern process automation, measuring devices or actuators are also used in areas at risk of explosion. This makes it necessary, in particular, that certain voltage, current or temperature values are not exceeded by the components used.
Thus, for example, the electric sensor systems for use in areas at risk for explosion are designed in the ignition protection category “intrinsic safety”. Hereby, intrinsic safety means that it is ensured that even in the case of malfunction, an unsafe situation will not arise, and in particular, there is no risk of explosion. Certain design requirements exist that are dictated, in part, by standards.
Voltages and currents occurring in the intrinsically safe electric systems are thus limited to safe maximum values in order to eliminate the risk of explosion due to spark ignition. Based on these values, a maximum value for power dissipation can be assumed in an intrinsically safe electric system.
Additionally, it also applies that the surface temperature of the electronic components arranged in the area at risk of explosion is limited in order to counter the risk of explosion due to hot surfaces. This can also occur by limiting power dissipation, for which the thermal resistance of the affected component is to be used.
So-called temperature classes, each having a maximum allowable temperature value, are given for explosion control. For example, the surface temperature may reach a maximum of 135° C. in the temperature class T4 classification.
Due to a relatively high thermal resistance in some components—such as the heating element of calorimetric mass flowmeters—the temperature limitation leads to the power dissipation being limited to a relatively small value.
Different arrangements for calorimetric or thermal flow measurement are known from the prior art. This type of determination of the flow of fluids—e.g., liquids or gases—in piping systems is suitable, in particular, for the measurement of low flow velocities. Thereby, the signal obtained from measurement is directly proportional to the mass flow. German Patent Application DE 10 2010 015 813 A1 and corresponding U.S. Pat. No. 8,544,352 B2 are referred to as an example of the prior art.
An essential measuring variable is the heat flux, which is given off from the heated element—electronic component—to the flowing fluid and taken away by the fluid. The heat flux taken away corresponds to the electric heat output.
In practice, two methods are generally discerned:
In the so-called cooling method, the flowing fluid cools the heating element that is inserted in the flow cross-section of the measuring tube and the temperature of the heating element is kept constant. The required heat output for generating the constant temperature difference is used for determining the mass flow.
In the so-called heating method, it is determined at which measure the heat flux taken away from the heating element increases the temperature of the medium at a second, downstream sensor. Alternatively, the increase in temperature can be kept constant by readjusting the heat output, so that then the required heat output is used as measuring variable for the mass flow.
Thus, the measuring principle requires a heating component.
In the case that no fluid is present or that the flow is too small, it is, in particular, necessary that it be ensured that the heating element, which is also called a heater, does not become heated over an allowed temperature value.